1. Field of the Invention
This invention relates generally to the field of mass storage disk devices, and more specifically to apparatus in a disk controller which enables the controller to get into byte synchronization with a serial stream of bits received from a disk device during a disk read operation.
2. Description of the Prior Art
A typical data processing system may include a main memory and a central processor unit (CPU) coupled to a disk controller which interfaces to a number of disk devices. Each disk device includes a number of disks mounted to a rotating shaft. Each disk surface is traversed by a magnetic head which may be indexed in a radial direction to fixed positions to read or write information which is stored in the magnetic material coated on each disk surface. Information is stored in concentric tracks on the surface of each track.
Each of the tracks is organized in sectors. Each sector includes address information identifying the sector and data byte information. Both the address information and the data byte information are preceded by a number of ZERO bytes followed by a synchronization character, followed by the information.
When the disk controller addresses a track on the disk, the disk controller receives a serial stream of data bits on a data signal line and a stream of clock bits to condition the disk controller to identify the data bits. Information is processed in the form of data bytes. Therefore, the stream of data bits must be organized into data bytes. The disk controller, therefore, tries to identify the synchronization byte in order to condition the disk controller to separate the data bit stream into data bytes.
In addition to the sector address and data byte information, the disk device may include defective track information which is written on one surface of a disk of the disk device. Defective track information includes those locations on the surface of a disk upon which information may not be stored. This may be caused by imperfections in the magnetic coating on an area of a disk surface.
This defective track information is written on a predetermined track sector by the manufacturer of the disk device when the disk device is tested. This defective track information must be preceded by a synchronization byte which may or may not be the same as the synchronization bytes preceding the sector address and data byte information. Also, different manufacturers may use different synchronization bytes.
The prior art disk drives included the logic for detection of the synchronization byte in the logic. To change the logic to recognize another synchronization byte required a hardware change which was costly, time-consuming and also required that records be kept of which disk controller could process information on which disk drive.
This also presented a problem of processing disk packs (replaceable by the user) which had been written on another data processing system using a different synchronization character.